This invention relates to a fuel vapor processing device for vehicles.
JP5-47403A published by the Japanese Patent Office in 1993 discloses a pressure control valve in the middle of a vent passage which sends the fuel vapor in a fuel tank for vehicles into a canister. The canister contains a fuel adsorption material such as activated carbon, and the fuel vapor in the fuel tank flows into the canister through a vent passage according to the pressure buildup of the fuel tank, and is adsorbed by the adsorption material. The canister is provided with a vent pipe which communicates with the atmosphere.
When an intake passage goes to negative pressure due to operation of an engine, the fuel vapor is drawn out of the adsorption material, aspirated into the intake passage via a purge passage together with the air introduced from a vent pipe, and burnt in the engine.
If the vent pipe becomes clogged, the negative pressure of the intake passage acts directly on the fuel tank via a purge passage and vent pipe due to the operation of the engine, and if this state continues for a long time, the fuel tank may deform. According to the prior art, the pressure control valve closes the vent pipe upon detection of a negative pressure in the vent pipe so as to prevent the negative pressure from acting on the fuel tank.
In such a fuel vapor processing device, it is necessary to occasionally check whether or not a fuel leak has occurred in the fuel vapor path, and a leak diagnosis is performed for this purpose. The diagnosis is carried out by closing the vent pipe of the canister, introducing a negative pressure from the intake passage to the fuel tank, and closing a purge valve provided in the purge passage so as to seal the section from the fuel tank to the purge valve. In this state, the pressure of the sealed section is monitored, and the presence of a leak is diagnosed from a pressure change.
However, if a pressure control valve is provided in the middle of the vent passage as in the prior art, it is impossible to apply a negative pressure to the fuel tank, and to diagnose a fuel leak. In order to enable fuel leak diagnosis, the pressure control valve must have a special mechanism so that communication between the fuel tank and canister is maintained even under a negative pressure during leak diagnosis. This complicates the construction of the fuel vapor processing system and leads to increased manufacturing costs.
It is therefore an object of this invention to prevent the action of an excessive negative pressure on a fuel tank while enabling leak diagnosis in a simple construction.
In order to achieve the above object, this invention provides a fuel vapor processing device for use with an engine which burns air from an intake passage and fuel supplied from a fuel tank. The device comprises a canister which adsorbs fuel vapor in the fuel tank, a first passage which connects the fuel tank and the canister, a second passage which connects the canister and the intake passage. The canister has an air vent which communicates with the atmosphere and a purge valve is installed in the second passage. The device further comprises a pressure sensor which detects a negative pressure in a section leading from the purge valve to the fuel tank via the first passage, canister and second passage, and a programmable controller which controls the opening of the purge valve according to the negative pressure.
The controller is programmed to compare the negative pressure, when the purge valve is open, with a reference negative pressure, and decrease the opening of the purge valve when a magnitude of the negative pressure is larger than a magnitude of the reference negative pressure.
This invention also provides a control method of a fuel vapor processing device for use with an engine which burns air from an intake passage and fuel supplied from a fuel tank. The fuel vapor processing device comprises a canister which adsorbs fuel vapor in the fuel tank, a first passage which connects the fuel tank and the canister, and a second passage which connects the canister and the intake passage. The canister has an air vent which communicates with the atmosphere and a purge valve is installed in the second passage.
The control method comprises detecting a negative pressure in a section leading from the purge valve to the fuel tank via the first passage, canister and second passage, comparing the negative pressure, when the purge valve is open, with a reference negative pressure, and decreasing an opening of the purge valve when a magnitude of the negative pressure is larger than a magnitude of the reference negative pressure.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.